1. Field of the Invention
The present invention relates to a magnetic recording and reproducing apparatus such as VTR adapted to detect the occurrence of clogging of a head.
2. Description of the Prior Art
When video heads get clogged, efforts to perform recording on a magnetic tape may result in imperfect recording quality or become impossible. If such an imperfect magnetic tape on which recording has been performed is used for reproduction, even with a normal VTR having video heads that are not suffering from clogging, the reproduced image may frequently include noises and it is even possible that no images at all will be reproduced.
Conversely, even if a magnetic tape which has been properly recorded is reproduced by means of a video head which is clogged, many noises may similarly be generated and on occasion no image will be reproduced.
The majority of the causes of clogging are attributed to the fact that the magnetic powder generated by a magnetic tape may stick to the gaps defined by the video heads, inhibiting formation of a normal magnetic circuit. This is inevitable with a VTR or the like.
Such clogging may be easily removed by cleaning the tip of a video head with a cleaning liquid or having a cleaning tape run along said tip.
It is to be understood, however, that the clogging may be easily observed on the monitor screen during reproduction, but clogging is difficult to detect during recording. When it is desired to detect the presence of clogging during recording, recording has to be suspended and the reproduction mode tested in order to detect any clogging.
In particular, video tape recorders used for surveillance for long periods of time (hereafter referred to as "time-lapse VTR") according to the prior art are assumed to continue recording in an unmanned mode all day long, or for months on end in some cases. Thus it is quite unlikely to be possible to reproduce a magnetic record to confirm whether proper recording has definitely been effected. Consequently, it often happens that the subject matter to be recorded is not properly recorded, which is a critical problem with this kind of VTR when so-called "clogging" occurs as a result of adherence to video heads of magnetic powders or the like from the surface of video tapes. This makes it impossible to properly record and reproduce records. As countermeasures directed at solving this problem, not only has it been the practice for users to frequently check the condition of reproduced images and/or to have a specialist periodically check for the presence of "clogging", but there has also been a practice of attaching a head cleaning mechanism as shown in FIG. 1 in order to clean the video heads regardless of whether "clogging" actually occurs or not.
This cleaning mechanism according to the prior art will now be explained by referring to FIG. 1. The numeral 1 in FIG. 1 designates a rotary drum, numeral 10 a tape guide, numeral 2 a rotary roller for cleaning, numeral 11 a support arm for the rotary roller, numeral 12 a spring for pressing, numeral 13 a cam for moving the support arm and numeral 14 a pin for moving the support arm 11.
Operation of the above cleaning mechanism will next be explained by referring to FIGS. 2 through 4. When the tape guide 10 commences movement in the direction indicated by an arrow d.sub.1 in FIG. 2 for the purpose of recording or reproduction, the cam 13 is caused to rotate synchronously in the direction indicated by an arrow d.sub.2 and the rotary drum 1 is also caused to start rotating. As shown in FIG. 3, the roller 2 is urged against the rotary drum 1 by the biasing spring 12 while the tape guide 10 is moving so that the video head may be cleaned by the roller. At the position where the tape guide 10 completes this movement, the pin 14 will move the support arm 11 against the force of the biasing spring 12, as shown in FIG. 4, whereby the roller 2 is moved away from the rotary drum 1.
In the magnetic recording and reproduction apparatus of prior art, since the video head cleaning mechanism is so constructed that the roller is urged against the head at the time of loading or unloading a tape in the manner described above, and that the head will be cleaned at the time of and with the same frequency as loading regardless of whether the heads are clogged or not, there have been certain problems in that recording cannot always be performed as soon as is necessary, and in that cleaning is performed so often that the heads may be damaged.